FCM file format description
FCM file consists of a variable-length header and various blocks that depend on settings. The header must be a minimum of 56 bytes. The first 32 bytes are interpreted the same between v1 and v2 movies. The header and file layout are completely different between blip's patched version and the unpatched official version of FCEU. Only the savestate and controller data are the same.

Header format:

    000 4-byte signature: 46 43 4D 1A "FCM\x1A"
    004 4-byte little-endian unsigned int: version number, must be 2
    008 1-byte flags:
       bit 0: reserved, set to 0
       bit 1:
           if "0", movie begins from an embedded "quicksave" snapshot
           if "1", movie begins from reset [1]
       bit 2:
           if "0", NTSC timing
           if "1", PAL timing
           see notes below
       other: reserved, set to 0
    009 1-byte flags: reserved, set to 0
    00A 1-byte flags: reserved, set to 0
    00B 1-byte flags: reserved, set to 0
    00C 4-byte little-endian unsigned int: number of frames
    010 4-byte little-endian unsigned int: rerecord count
    014 4-byte little-endian unsigned int: length of controller data in bytes
    018 4-byte little-endian unsigned int: offset to the savestate inside file
    01C 4-byte little-endian unsigned int: offset to the controller data inside file
    020 16-byte md5sum of the ROM used
    030 4-byte int: version of the emulator used
    034 name of the ROM used - UTF8 encoded nul-terminated string.

After the header comes "metadata", which is UTF8-coded movie title string. The metadata begins after the ROM name and ends at the savestate offset. This string is displayed as "Author Info" in the Windows version of the emulator.

The savestate offset is <header_size + length_of_metadata_in_bytes + padding>. The savestate offset should be 4-byte aligned.
At the savestate offset there is a savestate file.
The savestate exists even if the movie is reset-based.

The controller data offset is <savestate_offset + length_of_compressed_savestate + padding>. The controller data offset should be 4-byte aligned.
The controller data contains <number_of_frames> frames.

Note that the savestate data must come before controller data.
A gap between the savestate data and the controller data is allowed.

Starting with version 0.98.12 released on September 19, 2004, a PAL flag was added to the header but unfortunately it is not reliable - the emulator does not take the PAL setting from the ROM, but from a user preference. This means that this site cannot calculate movie lengths reliably.

The controller data is written as follows: First there is an "update" byte, followed by zero to three "delta" bytes.

Update byte:

  76543210
  x.......    if "1" this is a "control" update, if "0", this is a "controller" update.
  .xx.....    specifies the number of delta bytes to follow
  ...xxxxx    update data.

The "delta" is the number of emulator frames between this update and the next update. It is encoded in little-endian format and its size depends on the magnitude of the delta:

  Delta of:      Number of bytes:
  0              0
  1-255          1
  256-65535      2
  65536-(2^24-1) 3

FCEU emits a "dummy" command of 0 if the current delta is about to overflow.

The interpretation of the update data field depends on whether is a "control" or "controller" update. The "control" commands are listed as follows:

   0     Take no action
   1     Reset
   2     Power cycle
   7     VS System Insert Coin
   8     VS System Dipswitch 0 Toggle
  24     FDS Insert
  25     FDS Eject
  26     FDS Select Side

The controller update data is formatted as follows

  43210
  xx...  the gamepad number the update applies to.
  ..xxx  the specific input to toggle.

The inputs are numbered as follows:

  0      A button
  1      B button
  2      Select button
  3      Start button
  4      Up
  5      Down
  6      Left
  7      Right

Controller update data is emitted to the movie file only when the state of the controller changes.

[1]: Even if the header says "movie begins from reset", the file still contains a quicksave, and the quicksave is actually loaded. This flag can't therefore be trusted. To check if the movie actually begins from reset, you'll have to analyze the events and see if the first non-idle command in the file is a Reset type control command. 